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ost human societies around the globe eschew inbreeding, and for good reason. Mating with close relatives can lead to disabling, and even deadly, genetic disorders. Historians, for example, believe that Charles II of Spain (1661–1700), who suffered from numerous excruciating physical and mental health problems, was inbred. Impotent, he died without an heir, ending the Spanish Habsburg dynasty.

The prehistoric Homo sapiens that migrated out of Africa likely formed mating networks to avoid such problems, suggests a new investigation of genomes belonging to four individuals who were buried at around the same time 34,000 years ago in Sunghir, Russia. The findings were published Oct. 5 in the journal Science.

“While we did find evidence for relatively small recent effective population sizes of around 300 individuals, we found no signs of inbreeding,” lead author Martin Sikora of the Center for GeoGenetics at the University of Copenhagen told Seeker.

Sikora and his team made the determination after analyzing the genomes of 4 of 9 known buried individuals associated with the Sunghir site. One was an adult male, whose skeleton is complete. Two others were juveniles that were interred head to head. The fourth consisted solely of an adult femur that had been filled with ochre. That bone, likely processed in a burial ritual of some kind, had been placed next to a shoulder of one of the juveniles.

Their DNA shows that the individuals were not closely related in genetic terms. At the most, the people were second cousins. This was even true for the two juveniles who were buried so closely together in the same grave.

Evolved forms of kin recognition, such as olfactory cues, help non-human primates and other animals to avoid inbreeding. Co-author Marta Mirazón Lahr of the Leverhulme Center for Human Evolutionary Studies at the University of Cambridge explained that most non-human primate societies are also organized around single-sex kin, where one of the sexes remains resident and the other migrates to another group.

“At some point, early human societies changed their mating system into one in which a large number of the individuals that form small hunter-gatherer units are non-kin,” Lahr continued. “The results from Sunghir show that Upper Paleolithic human groups could use sophisticated cultural systems to sustain very small group sizes by embedding them in a wide social network of other groups.”

Archaeological evidence, such as tools, support that the Sunghir individuals were part of a larger mid-Upper Paleolithic industry called the Gravettian, which lasted 17-000-34,000 years ago. Multiple sites around Western Eurasia, such as Dolni Vestonice in the Czech Republic, have revealed clues about this cultural group.

Three 31,000-year-old skulls from Dolni Vestonice in the Czech Republic |
Martin Frouz and Jiří Svoboda

The researchers believe the social networks of these early people were probably similar to those of modern hunter-gatherers, such as Aboriginal Australians and Native Americans. Like their prehistoric ancestors, these existent people may live in small groups of around 25 people, but are then less directly connected to a larger community of about 300.

The ochre-filled bone is not the only evidence for possible early rituals among the Gravettian. The Sunghir graves contained numerous ornaments and other items including beads made of mammoth ivory, stone pendants, strings of Arctic fox canine teeth, and more. It is believed that the adult male was wearing heavily adorned clothing, with beads sewed onto both his inner and outer garments.

“The rich cultural goods found in the burials clearly suggest that these people performed ceremonies and rituals associated with important life events,” said Sikora. The ceremonies may have accompanied the exchange of mates between groups.

Prior genetic research on a 45,000-year-old individual from Ust’Ishim, Siberia, showed the individual had also not been inbred, so the origin of the strategic social structures could go back much further in time.

Previously it was thought that inbreeding was prevalent among Neanderthals, given the genome of one hominid from the Altai Mountains of Siberia. A more recent study of Neanderthals from Vindija Cave, Croatia, who lived around 52,000 years ago showed that they were not inbred.

The Altai inhabitants might have inbred due to unintended isolation, possibly resulting from the region’s challenging climate or from other unknown reasons.

The ancestors of the Sughir people mated with Neanderthals, according to the genetic evidence.

Sikora said that he and his team estimated the fraction of Neanderthal DNA in the Sughir individuals’ genomes to be slightly higher than in present-day people, which would be consistent with natural selection purging some Neanderthal DNA over the past 34,000 years.

“We also used our data to obtain a refined date for the Neanderthal admixture event, which we estimate happened sometime between 52,000 and 63,000 years ago,” he said.

By then, anatomically modern humans had already been in Europe and Asia. It is believed that the African ancestors of the Gravettian people migrated out of Africa between 70,000–100,000 years ago. By at least 45,000 years ago, they had reached western Siberia. By 31,000 years ago, and perhaps even earlier, they were in far northern Siberia as well as Australia.

“I do find it quite remarkable that our ancestors were apparently able to migrate so efficiently and successfully across these vast landscapes that clearly were also environmentally very challenging,” Sikora said.

“That these groups were able to maintain the genetic diversity we observe and avoid inbreeding despite being in these sometimes harsh and isolated environments is fascinating,” he added. “It does suggest a level of interconnectedness that could be one of the key reasons why our species has been so successful in colonizing every continent to even the remotest corner.”

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